Alloy

ABSTRACT

A COBALT-BASE METAL ALLOY WITH SUPERIOR HIGH TEMPERATURE PROPERTIES.

United States Patent 3,592,638 ALLOY William R. Freeman, Jr., Easton, Conn., assignor to Avco Corporation, Stratford, Conn. No Drawing. Filed Aug. 22, 1969, Ser. No. 852,499 Int. Cl. C22c 19/00 US. Cl. 75--171 11 Claims ABSTRACT OF THE DISCLOSURE A cobalt-base metal alloy with superior high temperature properties.

This invention relates to a cobalt-base metal alloy with superior high temperature properties and to the heat treatment of such alloy to develop significant improvements in the mechanical properties exhibited by the alloys when subjected to high temperatures and stresses in service.

A principal object of this invention is to provide a cobalt-base alloy which may be cast to desired shapes, such as turbine blades, vanes, and other parts used in high temperature gas turbine engines.

Gas turbine engineering and its associated technology have made immense advances since their inception in the late 1930s With the technology of materials development and its application being foremost. The requirements of materials for use in the turbine section are currently being met by nickel-base superalloys due to their high temperature strength coupled with good inherent oxidation resistance.

The principal strengthening mechanism for high temperature nickel-base alloys is the precipitation of the secondary phase Ni (Al, Ti). The nickel-base superalloys based on this hardening mechanism are somewhat unique in comparison to conventional age hardening alloys due to the reversibility of the age hardening process. When heated to a higher temperature, the gamme prime Ni (Al, Ti) hardened nickel alloy shows some loss of hardness. However, when cooled back down to lower temperature, reprecipitation restores the hardness to its initial level.

The other major strengthening mechanism for nickel alloys is solid solution hardening, and additions of tungsten, molybdenum, and chromium are known to be effective in this respect. In addition, to solid solution hardening, chromium is considered to be the most etfective single element in imparting sulfidation resistance to nickelbase alloys. Unfortunately, any increase of chromium in a supersaturated alloy must be done at the expense of the more potent hardening elements, such as tungsten, molybdenum, aluminum or titanium, or combinations thereof. Thus, increasing chromium content will result in lower rupture strength even if the saturation of the nickel matrix is kept at the optimum level.

Cobalt-base alloys, on the other hand, have achieved wide usage in high temperature applications where moderate strength at elevated temperatures along with outstanding resistance to oxidation, thermal shock and thermal fatigue are required. Unlike the age hardened nickel-base alloys presently used, cobalt-base alloys may contain relatively high chromium contents (20-25 weight percent) to impart strength as well as oxidation resistance.

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The considerably higher chromium contents which are permissible in cobalt-base alloys are considered to be a major factor in the excellent resistance to sulfidation exhibited by such alloys.

Two mechanisms for enhancing the strength exhibited by such alloys at elevated temperatures have been utilized in cobalt-base alloy development. One is solid solution strengthening in which refractory metals, such as molybdenum, tungsten, and chromium are dissolved in the cobalt matrix. The other is precipitation hardening which is brought about by the formation and dispersion of metal carbides throughout the matrix of the alloy. Their formation can occur during solidification, by precipitation from solid solution, and/or by conversion. The type of carbides formed in cobalt-base alloy systems depends on the carbide forming strength and level of the alloying elements used and on the alloy carbon level. Some empirical rules have been developed in an attempt to achieve a proper balance between carbon and the carbide forming elements. For example, a ratio of approximately 0.7 for atomic percent carbon to the sum of the atomic percents of tung sten, tantalum, columbium, titanium and zirconium has been suggested for optimum high temperature rupture strength.

Because of this basic difference in hardening mechanisms, the strongest cobalt-base alloys are usually weaker than the strongest nickel-alloys. From the standpoint of hot corrosion (sulfidation) resistance, however, the cobaltbase alloys are significantly better than the present cast nickel-base alloys, apparently due to their inherently higher chromium contents.

Consequently, it would be very desirable to provide a cobalt-base alloy with improved mechanical and chemical properties and thereby produce a corrosion resistant alloy with high temperature properties comparable to a nickelbase alloy, such as Inco 7130.

This desirable objective is achieved according to the present invention by providing cast cobalt-base alloys of the following nominal analysis in weight percent: C, .70/.90; Mn, 10; Si, .10; Cr, 20/265; Ni, 9/12; W, 6/8; Ta, 2/8; Fe, 30; Co, balance.

Within the above ranges a more limited range of compositions according to this invention comprises: C, 0.75/ 0.90; Cr, 2450/2650; Ni, /115; W, 7/8; Ta, 4/8; Co, balance except for incidental impurities.

The most preferred alloys of the present invention, within the above noted analysis ranges, are those, containing about 0.85% C and approximately 4 to 8% Ta.

A further manner of achieving the desired objective is by heat treating alloys of the above noted compositions in order to enhance their high temperature mechanical properties.

The invention will be further understood from the description which follows taken in conjunction with the data presented therein.

The alloys evaluated were vacuum melted and vacuum cast into /1 inch ASTM Test bars such as those specified for stress rupture tests according to ASTM Standard E 59T.

All test bars were radiographed and then fluorescent dye penetrant inspected. Each alloy was analyzed for all the alloying elements plus likely impurities, after casting into test bars. The elements, chromium, nickel, tungsten and iron were analyzed by using X-ray fluorescent techniques; carbon manganese, and silicon by wet chemistry methods; columbium and tantalum were analyzed using both techniques.

Table 1 gives the composition of alloys according to the present invention and certain experimental alloys with which they were compared.

TABLE 1.CHEMICAL ANALYSIS OF EXPERIMENTAL ALLOYS 4 carbide being replaced by tantalum and/or columbium carbides and thus freeing the chromium for solid solution hardening.

Stress-rupture tests were conducted on all eight modifications of the basic alloy in the as-cast condition at 1500 F. and 1700 F. using stresses of 35,000 psi. and 20,000 p.s.i., respectively.

[Inweightpemnt] TABLE 2.-sTREss-RU1 TURE RESULTS FOR AS-CAST Alloy 0 Mn s1 Cr Ni w Cb 'Ia Fe 00 ALLOYS OF TABLEI LDAI Tested at 1,500 F. and 35,000 p.s.i. Tested at 1,700 F. and 20,000 psi.

200. 0.35 .00 0.01 20.34 11.34 7.34 0.10 0.10 .17 Bal. stress stress 202--- 0.72 .05 0.02 25.07 11.13 7.42 0.10 2.11 .10 1101. 204-1. 0.74 .03 N11 25.30 11.31 0.54 0.10 4.25 .10 301. Life, Elong., Life, El0ng., 204-2. 0.74 .04 N11 24.17 11.2 1 7.47 0.10 4.87 .31 Bal. hrs, percent Alloy hrs percent 204-3- 0.73 .03 0.03 22.35 11.27 7.53 0.10 4.00 .17 1151. 203-1. 0.73 Nil 0.03 23.70 0.45 7.00 0.31 .05 1351. 2032. 0.70 .03 0.03 24.30 0.04 7.11 0.10 10.13 .30 13-51. 203-3- 0.30 .04 0.04 23.03 0.21 7.03 0.10 10.33 .17 Bal. 220-1- 0.72 .00 0.05 25.32 10.50 7.24 2.00 0.10 .05 1351. 220-2. 0.75 .05 0.05 24.03 11.21 7.13 2.25 0.10 .10 130.1. 222--. 0.70 Nil 0.04 25.14 0.74 7.51 1.00 2.03 .05 1351. 240--- 0.73 .01 0.00 24.30 10.03 7.53 4.00 0.10 .10 1301. 244-1. 0.70 .05 0.02 22.02 10.04 7.01 3.53 4.50 .05 Bal. 244-2- 0.70 N11 0.03 21.50 11.37 0.75 3.55 4.14 .13 1351. 230- 0.75 N11 0.00 23.22 0.35 7.00 7.00 0.10 .10 Bal.

lographic examination revealed that the as-cast alloy consisted of a cobalt rich solid solution in which the carbide phases appeared to be present as M C and MI1C3 type carbides. A pepper type precipitate noted appeared to be Cr C carbide. The addition of either tantalum or columbium to the base alloy at levels above 2 weight percent produced a dendritic or script-like MC phase, in which M is primarily tantalum or columbium. The microstructures for both of the 4 and 8 percent tantalum and/ or columbium modified alloys also contained this phase. However, the amount of MC script is greatest in the 8 percent tantalum alloy. A lighter eutectic appearing area noted in the tantalum and columbium containing alloys appeared to be the M C type carbide in which M is chromium and/or tungsten. Hardness data were obtained with a Rockwell hardness tester for all modified alloys in the as-cast condition to determine the content of solid solution and carbide precipitation hardening due to the addition of columbium and tantalum addition. This data is summarized in Table 1A which follows.

TABLE L L-ROCKWELL C HARDNESS, AS-CAST As will be seen from the data in Table 1A, there is a definite trend of increasing hardness with the increasing alloying element with columbium giving higher hardness values than the tantalum at the same weight percent level. The more potent effect of columbium is logical considering that the metallurgical reactions occur on an atomic percent basis. The columbium has atomic weight of 92.91 where tantalum atomic weight is 180.95. Hence twice as many columbium as tantalum atoms will be present in the alloy for any given weight percent added. This increase in hardness is probably associated with the increased volume percentage of carbides which have resulted from the addition of tantalum and columbium along with the increase in carbon content of the alloy as compared with prior art alloys. Other factors which may account for the higher hardness are solid solution hardening by increased matrix chromium, resulting from some of the chromium 107. 0 16. 187. 6 22. 153. 0 20. 201. 0 20. 138. 9 15. 222. 3 20. 71. 6 14. 59. 9 20. 90. 0 20. 96. 1 25. 36. 8 25. 48. 2 26. 39. 2 BO 71. 6 BO 12. 7 17. 13. 5 20. 80. 8 15. 138. 9 13. 38. 7 15. 80. 2 20. 38. 6 10. 55. 8 20. 63. 7 30.

The stress-rupture results on all eight modified alloys at 1500 F. and 1700 F. using stresses of 35,000 psi. and 20,000 p.s.i. respectively are presented in Table 2. From the data it will be seen that additions of columbium plus tantalum gave moderate increases in the 1500 F. stress-rupture lives of the base alloy with the exception of high columbium addition (8 percent) which had detrimental effect on the rupture properties of the base alloy. The stress-rupture results at 1700 F. showed that the 4 percent tantalum addition improved the stressrupture life of the as-cast base alloy, but the 8 percent tantalum addition had little effect. Based on these results, it appears that 4 and 8 percent tantalum alloys possess the best stress-rupture properties of the alloy ranges.

It was found that the properties of these alloys could be further improved by heat treating the alloys at appropria e temperatures, either to produce additional carbide precipitation and so to harden by aging, or to initially homogenize the alloys in order to diminish the segregation present in the as-cast alloy and then aging the alloy for carbide precipitation.

The heat treatments were as follows:

(a) Heat treated in air at 1325 F. for 50 hours and then air cooled.

(b) Heat treated in air at 1550 F. for 24 hours and then air cooled.

(c) Heat treated in argon at 2150 F. for 4 hours and then argon cooled followed by 1550 F. for 24 hours in air and then air cooled.

Rockwell hardness Alloy As-cast H.T. (a) H.T. (b) H.T. (c)

LDA:

(a), (b), and (0) refer to previously described heat treatments.

The effects of the above noted heat treatments on the stress-rupture life of the 4 and '8 percent tantalum containing alloys are listed in Table 3.

TABLE 3.STRESS-RUPTURE RESULTS FOR HEAT TREATED ALLOYS OF TABLE 1 [Tests at 1500 F. and 35,000 p.s.i. stress] Heat treatment 2150 F./4 hr., 1325 F./50 hr. 1550 F./24 hr. 1550 F./24 hr.

Life, Elong., Life, Elong., Life, Elong Alloy hr. percent hr. percent hr. percent LDA:

The straight aging treatment at 1325 F. for 50 hours yielding the greatest improvement in the stressrupture life of the 4 percent tantalum alloy; the average life being 230 hours against an as-case life of 135 hours. This heat treatment however did not improve the stressrupture life of the 8 percent tantalum modified alloy and yielded a 100 hours life at 1500 F. The 1550 F. for 24 hours aging treatment did not improve the life of the 4 percent tantalum alloy, however, a marginal improvement was produced on the 8 percent tantalum alloy at 1550 F. The 215 0 F./ 4 hour solution treatment followed by a 1550 F./ 24 hour aging treatment produced a slight improvement in the life of the 4 percent tantalum modified alloy. Presumably, higher temperature aging treatment (1550 F. vs. 1325 F.) would result in improved stressrupture properties at a higher test temperature (i.e. 1600-1700 F. range).

The stress-rupture properties obtained for the as-cast 4 and 8 percent tantalum alloys were approximately 135 hours which is a 238 percent and 93 percent improvement, respectively, over as-cast and aged (1325 F./50 hours) comparable tantalum free alloy. The optimum combination of the alloy (4 percent tantalum) and heat treat condition (1325 F. for 50 hours, air cool) resulted in an improvement in stress-rupture life of 236 percent when compared to the tantalum free alloy with the same heat treatment.

Tensile test data of both the 4 and :8 percent tantalum containing alloys are summarized in Table 4.

TABLE 4.TENSILE TEST RESULTS FOR 4AND 8 PERCENT TANTALUM ALLOYS AFTER HEAT TREATMENT AT 1325 F. FOR 50 HOURS Ultimate tensile 0.2% yield Reduction strength, strength, El0ng., of area, K s.i K s.i. percent percent Ultimate tensile strength, 0.2% yield strength and percent elongationlwere obtained for these alloys in the heat treated condition (1325 F./50 hours, air cool), at room, 900 F., 1400 F. and 1600 F. The average room temperature ultimate tensile strengths of the heat treated 4 and 8 percent tantalum containing alloys were 112,000 p.s.i. and 130,000 p.s.i. and the average 0.2% yield strengths were 88,500 p.s.i. and 92,500 psi, respectively. The higher tensile properties obtained with 8 percent tantalum alloy may be due to additional MC (TaC) type carbide precipitation. The average room temperature elongation for both of these alloys was 0.2 percent. The 4 percent tantalum alloy possessed higher ultimate tensile and yield strength properties than 8 percent tantalum alloy in the intermediate temperature range (300-1300 F.), but there was no significant difference in strength over the 1400 F. temperature. Average elongations rang ing from approximately 2 percent at room temperature to 10 percent at 1600 F. were obtained for both 4 and 8 percent tantalum alloys. Comparing these two alloys with vacuum melted and aged (1325 F./50 hours, air cool) tantalum free alloy it can be seen that the tensile properties of the heat treated 4 percent tantalum containing alloy is higher than aged tantalum free alloy in the intermediate temperature range (i.e. 500 F.1300 F.).

The addition of 2 to 8 percent tantalum to a (0.85 weight percent carbon) alloy of the composition indicated resulted in improved high temperature strength. The best combination of stress-rupture, tensile and thermal fatigue properties were achieved by the addition of 4 and 8 weight percent tantalum to the base alloy. Further, it will be noted that amounts of Ta in excess of 8% do not appear to improve the strength properties of the alloy and that amounts less than 2% are inefiective. Hence the range of Ta content appears to be critical and should be between 2% and 8%, about 4% being particularly preferred for Ta.

Having now described the invention in accordance with the patent statutes, it is not intended that it be limited, except as may be required by the appended claims.

I claim:

1. A cobalt-base alloy consisting essentially of the following elements, in approximate percentages by weight:

from above 0.70 to 0.90% of carbon 20 to 26% of chromium 9 to 12% of nickel 6 to 8% of tungsten 2 to 8% total of tantalum balance cobalt, except for incidental impurities.

2. The alloy of claim 1 wherein the carbon is approximately 0.85% and the tantalum is about 4%.

3. The alloy of claim 1 wherein the total of tantalum is between 4% and 8% by weight.

4. The alloy of claim 1 as a casting.

5. The alloy of claim 1 which has been heat treated to improve the properties thereof at high temperatures.

6. The alloy of claim 1 which has been aged to produce additional carbide precipitation.

7. The alloy of claim 6 which has been aged at a temperature of about 1550" F.

8. The alloy of claim 6 which has been aged at a temperature of about 1325 F.

9. The alloy of claim 1 which has been homogenized by heating to a high temperature and thereafter aged at a lower temperature.

10. The alloy of claim 9 which has been homogenized at a temperature of about 2150 F. and then aged at a temperature above about 1300 F.

11. A cobalt-base alloy consisting essentially of the following by weight: 0.75 to 0.90% of carbon, 24.50 to References Cited UNITED STATES PATENTS 3,432,294 3/1969 Wheaten 75-171 0 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 14832, 32.5, 158 

